1. Field of the Invention
The present invention concerns an apparatus and method for inerting a storage tank, e.g., a fuel tank, containing a combustible liquid, e.g., a hydrocarbon fuel, and having an ullage region containing oxygen or nitrogen and oxygen, e.g., air. In particular, the present invention concerns an apparatus and method which flows storage tank ullage gas through either (1) an oxygen-scavenging molecular sieve, to produce an oxygen-depleted return ullage gas, or (2) a nitrogen-scavenging molecular sieve which is regenerated by a purge gas to produce a nitrogen-enriched gas. The return ullage gas of case (1) or the nitrogen-enriched gas of case (2) is flowed to the storage tank ullage region to render the gas in the ullage region non-explosive.
2. Related Art
Storage tanks for combustible liquids, such as fuel tanks, have a free space, referred to as the “ullage region”, above the liquid level in the tank. Without treatment, the ullage region contains a mixture of combustible vapor (a vaporized portion of the combustible liquid) and air, the composition of which is dependent upon factors such as the temperature and pressure conditions within the tank. At certain oxygen concentrations and combustible liquid temperatures the combustible vapor/air mixture in the ullage region comprises an explosive mixture which may be ignited by a spark. For safety's sake, it is therefore necessary to maintain the ullage region oxygen concentration below that needed to sustain fire or explosion.
Although the following discussion applies to storage tanks for combustible liquids generally, the most commonly encountered situation is fuel tanks containing a hydrocarbon fuel. The safety of fuel tanks aboard aircraft is of particular concern and much of the following discussion is couched in those terms. The concentration of oxygen in the ullage region of a fuel tank is affected by a number of factors including depletion of fuel in the tank, a change in altitude of an aircraft, entry of air into the tank, and rapid pressure reduction in the ullage region. The latter may occur, for example, when an aircraft reaches high altitude in a short time after take-off. The fuel in the fuel tank contains dissolved oxygen (from air) which boils out of the fuel at the reduced pressure present in the ullage region at high altitude, thereby creating an undesired increase in the oxygen concentration in the ullage region. Oxygen is also brought into the fuel tank ullage region as its pressure increases during descent to lower altitude, or landing of an aircraft.
While there are other methods for controlling the amount of oxygen present in the ullage region, the most common method is referred to as fuel tank inerting, which is the introduction of an inert gas, such as nitrogen, into the ullage region of a fuel tank, thereby displacing at least some of the oxygen-containing ullage gas and maintaining the concentration of oxygen within the ullage region at a level low enough that the ullage gas is rendered non-explosive. In many cases, the inert gas used for fuel tank inerting is stored onboard an aircraft or vessel and then introduced into the fuel tank when it is required.